Catalytic recombination of radiolytic gases in thorium oxide slurries



nited States Patent Ofitlce dfi idfiii Fatented Aug. 7, 1952 3 048 474CATALYTIC nncoMnmArroN or nanrorrrrc eases IN rnonrnM oxrnn a: r s

My invention relates to a method of combining hydrogen and oxygen intowater and more particularly to combining these elements into water in anaqueous thorium oxide slurry. This application is a continuation of myco-pending application, Serial No. 701,938, filed December 10, 1957, nowabandoned.

Water, upon being subjected to significant amounts of nuclear radiation,is decomposed to produce a mixture of gaseous products comprised chieflyof a substantially stoichiometric mixture of hydrogen and oxygen. Thisradiolytic decomposition presents a serious problem in the operation ofaqueous homogeneous nuclear reactors of the type described in co-pendingapplication Serial No. 321,078, entitled Improved Neutronic ReactorOperational Method and Core System, filed November 18, 1952, now U.S.Patent 2,945,794, by C. E. Winters et al. The method of operation ofthis type reactor is also described in Aqueous Homogeneous PowerReactors, by R. BwBriggs and J. H. Swartout, published in volume III,Session 12A, Proceedings of the International Conference on the PeacefulUses of Atomic Energy, United Nations, 1956. In one embodiment of thistype reactor, an aqueous uranylsulfate fuel solution is circulatedthrough a central core and an aqueous thorium oxide slurry is circulatedthrough a blanket surrounding the core. By means of this arrangementbreeding is carried out, that is, fissionable U is produced in theblanket slurry by irradiation of fertile Th During the operation of thistype reactor the water, which may be either light or heavy, is subjectedto strong irradiation by gamma and beta rays, fission fragments andneutrons, resulting in decomposition of the water into hydrogen, whichmay be ordinary hydrogen or deuterium, and oxygen. The bubbling of thesegases, even in small amounts, may create a serious reactor controlproblem and result in erratic operation. In addition these gases uponaccumulation present an explosion hazard. For these reasons and in orderto maintain the liquid level of the system,

particularly in the case of costly deuterium, these gases must berecombined into water. Recombination of these gases may be accomplishedexternal to that portion of the system where the decomposition primarilytakes place or may be accomplished, at least in part, internally if somecomponent of the fuel or blanket catalyzes the recombination at asufficiently rapid rate.

Internal recombination of at least a portion of these gases isadvantageous by virtue of its minimizing the engineering problemsassociated With external recombiners. Internal recombination, however,involves the problem of securing a catalyst having sufiicient activityto recombine the gases as rapidly as they are formed. In the case of ablanket slurry containing approximately 500 to 1000 grams of thorium perliter as thorium oxide and 3 to 5 grams of uranium per liter as U0 thesebeing the concentrations expected to be employed, it is estimated thatfor satisfactory performance the catalyst should have an activity suchthat a reaction rate equivalent to the consumption of approximatelymoles of hydrogen per liter per hour is attained at a temperature of 280C. and a hydrogen partial pressure of 500 p.s.i. Recombination iscatalyzed to a limited extent by the slurry itself. For example, in athorium oxide slurry containing 530 g. of thorium per kilogram of waterand at a temperature of 300 C., hydrogen partial pressure of 500 psi.and in the presence of excess oxygen, hydrogen and oxygen are combinedat rates from 0.01 to 0.045 mole of H per liter per hour by the thoriumoxide alone and at rates from 0.08 to 0.01 mole H /l./hour when 4 to 8%uranium trioxide is added to the slurry. These rates are, of course,impractically low.

In addition to a satisfactory reaction rate, the catalyst must becompatible with other components of the system, must have a low thermalneutron capture cross section, and must be economically practical. Theserequirements have been met for a uranyl sulfate fuel solution by the useof copper ions as the catalyst, as described in copending applicationSerial No. 339,489, entitled Combination of Hydrogen and Oxygen, filedFebruary 27, 1953, now U.S. Patent 2,863,729, by Harold F. McDuflie andCharles H. Secoy. Copper ions, however, are not a satisfactory catalystin a thorium oxide slurry because of the absorption of these ions by thethorium oxide particles. Copper ions function as a solution catalyst,and their effectiveness is reduced with the decreased ionicconcentration resulting from adsorption.

An object of my invention, therefore, is to provide a suitable catalystfor combining hydrogen and oxygen in aqueous slurry systems.

Another object is to provide a catalyst for recombining hydrogen andoxygen produced .by radiolytic decomposition in aqueous slurry systems.

Another object is to provide a catalyst for internally recombininghydrogen and oxygen produced by radiolytic decomposition in a ThO slurryused as the blanket breeding material in a thermal breeder neutronicreactor.

Another object is to provide a hydrogen and oxygen combination catalysthaving a relatively low thermal neutron absorption cross section.

Another object is to provide such a catalyst which is chemicallycompatible with a thorium-uranium oxide slurry.

Other objects and advantages of my invention will be apparent from thefollowing description.

In accordance with my invention hydrogen and oxygen in an aqueousthorium om'de slurry may be combined by the addition of a small amountof molybdenum trioxide to the slurry. Substantial amounts of thehydrogen and oxygen produced by irradiation of a thorium-uranium oxideslurry in a nuclear reactor may be recombined internally by this means,thus minimizing the problems associated with external recombiners.Molybdenum trioxide is a suitable catalyst for thorium oxide slurriesboth with regard to thermal neutron absorption and chemicalcompatibility with the slurry. Although my invention is primarilyapplicable to the blanket slurry in a two-region homogeneous reactor, itshould be ap parent to one skilled in the art that a molybdenum trioxidecatalyst may also be employed under proper conditions in a one-regionreactor employing a thorium oxide slurry.

My invention is applicable generally to the combining of hydrogen andoxygen in pressurized aqueous thorium oxide systems over a wide range oftemperatures and concentrations. A rapid reaction rate may be obtainedwithin the to 300 C. temperature range expected to be employed in theblanket slurry of a nuclear reactor, although my invention is not to beunderstood as so limited. Substantial recombination may also be obtainedat temperatures as low as room temperature, and the increased reactionrate at high temperatures indicates that eliective combination could beobtained at temperatures above 300 C. A total system pressure of over200 p.s.i. is required for effective combination,

with the reaction rate increasing with increasing pressures. Pressuresof 200-3000 p.s.i. are generally satisfactory, and a particularlyeffective reaction rate may be obtained by maintaining a pressure levelof about 2000 p.s.i. as employed in most homogeneous reactors.

The concentration of molybdenum trioxide required for a sufficientlyrapid reaction rate varies with the calcination history of the thoriumoxide in the slurry. Thorium oxide calcined at a relatively hightemperature, that is, above 800 C. is preferred for reactor slurry usebecause of its desirable physical properties. In general thorium oxidecalcined at higher temperatures for relatively short periods of timerequires less catalyst than oxide calcined at lower temperatures forrelatively long periods of time. For example, a concentration of 0.012molal is sufficient for a slurry containing thorium oxide calcined at1600 C. for four hours but 0.025 molal is required for oxide calcined at900 C. for twenty-four hours. For the conditions expected to beencountered in the thorium oxide blanket slurry of a nuclear breedu erreactor, that is, at operating temperatures from 150 to 300 C. and atconcentrations of approximately 500 to 1000 grams of thorium and 3 to 5grams of uranium per liter, a molybdenum trioxide concentration of from0.01 molal to 0.2 molal is effective, depending on the calcinationhistory of the thorium oxide. While a slight amount of combination maytake place at concentrations lower than this range, the reaction ratemay be impractically slow. Concentrations higher than this range mayalso be employed within the scope of my invention, but reactorefiiciency would be unduly decreased. For a slurry containing oxidecalcined at 900 C., a concentration of 0.05 molal is preferred since amaximum of catalytic activity takes place at this concentration. Theoptium concentrations for higher temperature-calcined thorium oxides areslightly lower.

An increased combination rate may be obtained by activating the slurrywith hydrogen. In this procedure the slurry containing the catalyst isheated under a hydrogen overpressure in the substantial absence ofoxygen. Although these conditions are not critical to my invention,satisfactory activation may be obtained by heating the slurry containingthe catalyst for 1.5 hours at a temperature of 280 C. and a hydrogenover-pressure of 1000 p.s.i. measured at room temperature. Atconcentrations of 0.1 molal and below, the combination rate is usuallysubstantially increased by this means, but at higher concentrations therate is only slightly affected.

Prolonged heating of the slurry under an oxygen overpressure has nodetrimental ettect on the combination rate. Separate heating of theslurry under an oxygen atmosphere is not essential to the invention, butthis measure may be employed to reduce corrosion of the reactor vesselsby the slurry.

My invention is further illustrated by the following specific examples.

EXAMPLE I The combination reaction of hydrogen and oxygen in thoriumoxide and thorium-uranium oxide slurries without the presence of anyadded catalyst was tested in the following manner. Ten ml. of a slurrywas added to a ml. stainless steel bomb. Oxygen and hydrogen were addedunder pressure in stoichiometric ratio at room temperature, and the bombwas heated to 300 C. The decrease in pressure was measured by means of awater-filled capillary connected to the bomb and a pressure cell, whichin turn actuated a recorder. The moles of hydrogen, n, removed from thesystem per liter of slurry per unit time, 2, were calculated from theequation the interval, perfect gas behavior and first order depeudenceon hydrogen partial pressure being assumed. For a slurry containing 530grams of thorium as thorium oxide per liter, rates of 0.04 and 0.045mole H /liter/ hour were obtained. The rate was increased to 0.1 mole H/Iiter/hQur for a slurry containing 530 grams of thoritun oxide perliter and 4 percent uranium trioxide.

EXAMPLE II A series of runs was made to test the catalytic activity ofmolybdenum trioxide at various temperatures and concentrations. Theseruns were conducted in 15-16 ml. stainless steel bombs provided with aninternal thermocouple and a pressure connection through the bomb cap.Pressure and temperature in the bomb were measured and recorded by thethermocouple, a pressure cell and conventional electronicinstrumentation. The bomb was heated by an aluminum pipe wound with aninsulated Nichrome wire. The heater temperature was measured with athermocouple positioned between the bomb and heater walls and wasclosely regulated by commercial controls. The bomb and heater assemblywas mounted on the platform of a commercial shaker in order to agitatethe slurry.

Each of the runs was conducted in the following manner. The dry solids(thorium oxide, uranium oxide and molybdenum trioxide) were tumbled forone hour. The mixture was then heated and shaken in water at 280 C. forthree hours in the presence of an overpressure of oxygen. At eachmolybdenum trioxide concentration listed in Table I a portion of theslurry was heated to 270-280 C. for 1.5 hours under hydrogen at 1000p.s.i. measured at room temperature. At each concentration except 0.05molal a portion of the slurry which had been heated under hydrogen wasthen heated at 270280 C. for 2 hours under 500 p.s.i. oxygen, meas uredat room temperature. In each combination run the bomb was filled toslightly over half its capacity with slurry at room temperature andcharged with a mixture of hydrogen and oxygen under pressure with theoxygen being slightly in excess of the stoichiometric amount. The bombwas heated to operating temperature and the total pressure recorded atregular intervals. The moles of hydrogen, n, removed from the system perliter of slurry per unit time, t, were calculated by the method ofExample I. Results of these runs are listed in Table I.

Table I COMBINATION RATES OF HYDROGEN AND OXY DN I THORIUM OXIDESLURRIES A'l VARIOUS ISEOLY DENUM OXIDE CONCENTRATIONS [Slurryz 900 C.calcined Th02, 1,000 g. of Th per kilogram of H20 0.5 mole percentUOa-I-IeO M003; heated for 3 hrs. at 280 C. under 02 (250-300 p.s.i. atroom temperature)] Hzcombl- MOO3 Ten-warm nation rate conceutra-Conditions ture k PH2=500 tion C.) p.s.i. (molal) (moles/1m] liter) 0.05As prepared 243 7 3 Heated for 1.5 hsr. 40 11 12 under H: at 270- 2800.. 0.10 As prepared 276 3 1 Heated for 1.5 hrs. 21 16 under 112 at 270280 C. Reheated reduced 206 67 31 slurry for 2 hrs. under 02 at 270- 280C. 0.15 As prepared 281 21 5 Heated for 1.5 hrs. 243 25 9 under H2 at270- 280 C. Reheated reduced 253 19 6 slurry for 2 hrs. under 02 at 270-280 C? 0.20 As prepared 264 27 10 Heated or 1.5 hrs. 16 10 under H2 at270- 280 0.

1 1,000 p.s.i. H2 at room temperature. 2 500 p.s.l. 02 at; roomtemperature.

5 EXAMPLE III A second series of runs was made, using the procedure ofExample II. In this series the combination runs were conducted at 150and 280 C. Details are listed in Table II.

Table II HYDROGEN-OXYGEN COMBINATION RATES IN THORIUM-URANIUM OXIDESLURRIES [Slurryr thorium, 1,000 g. per kg. of H30, Th0; calcined at900C.;

1 Heated with hydrogen overpressure for 1.5 hrs. at 280 0.; 1,000 p.s.i.added at room temperature,

2 Extrapolated from rate data obtained at 250 C. and below.

3 Extrapolated from rate data obtained at 102 C. and below.

EXAMPLE IV A thorium-uranium oxide slurry having a concentration of 750grams Th/kg. H O, 0.5 mole percent enriched U 30 and 0.15 molal M00 wasirradiated in a nuclear reactor at 300 C. for 192 hours at a powerdensity of 4.3 kw./liter, this being within the range of power densitiesexpected in large scale thermal breeder reactors. Less than 100 p.s.i.gas pressure in excess of steam pressure existed after the irradiation.This result was obtained for a slurry which had not been activated withhydrogen. This low increase in pressure under these conditions indicatesthat substantial recombination was eflected.

EXAMPLE V The recombination rates of hydrogen and oxygen in slurriescontaining thorium-uranium oxides calcined under various conditions weredetermined in a series of runs using the procedure of Example II. Ineach run the slurry contained 500 grams thorium in the form of thoriumoxide per liter and 0.5 percent uranium as uranium trioxide. Themolybdenum trioxide concentration in each run except the last was 0.05molal. The details of these runs are listed in Table III.

Table III EFFECT OF OALCINATION TEMPERATURE AND TIME ON HYDROGEN-OXYGENCOMBINATION IN THORIUM OXIDE SLURRIES Thorium-uranium Moles Hz per hourper liter slurry (H; partial oxide calcination pressure 500 p.s.i.)

conditions Slurry as prepared Activated slurry 1 Temp. Time 0.) (hours)Moles Reaction Moles Reaction temp. temp. 0.) C.)

1 Slurry heated under H: (250 p.s.i. measured at 25 C.) for 2 hours at270 C.

"Molybdenum trioxide concentration 0.012 molal.

The above examples are merely illustrative and are not to be understoodas limiting the scope of my invention, which is limited only asindicated in the appended claims. In addition my invention is applicablegenerally to pressurized thorium oxide and thorium-uranium oxideslurries and is not to be understood as limited to a specific nuclearreactor design.

Having thus described my invention, I claim:

1. A method of combining hydrogen and oxygen in an aqueous slurrycontaining a major proportion of thorium oxide and a minor proportion ofuranium trioxide which comprises providing a small amount of molybdenumtrioxide in said slurry under superatmospheric pressure.

2. The method of claim 1 wherein said molybdenum trioxide is provided ata concentration within the range of 0.01 molal to 0.2 molal.

3. A method of combining hydrogen and oxygen in an aqueous slurrycontaining a major proportion of thorium oxide and a minor proportion ofuranium trioxide which comprises heating said slurry under a hydrogenatmosphere and providing a small amount of molybdenum trioxide in saidslurry under superatmospheric pressure at a concentration within therange of 0.01 molal to 0.2 molal.

4. A method of combining hydrogen and oxygen resulting from thesubjection of an aqueous slurry containing a major proportion of thoriumoxide and a. minor proportion of uranium trioxide to ionizing radiationin a nuclear reactor which comprises providing molybdenum trioxide insaid slurry at a concentration within the range of 0.01 molal to 0.2molal under superatmospheric pressure.

5. The method of claim 4 wherein the temperature of the resultingmolybdenum trioxide-containing slurry is within the range ofapproximately 150 C. to 300 C.

6. The method of claim 4 wherein said thorium oxide is calcined at atemperature within the range of approximately 900 C. to 1600 C. beforesaid thorium oxide is added to said slurry.

References Cited in the file of this patent UNITED STATES PATENTSOhlinger et al Apr. 24, 1956 McDufiie et al. Dec. 9, 1958 OTHERREFERENCES

1. A METHOD OF COMBINING HYDROGEN AND OXYGEN IN AN AQUEOUS SLURRYCONTAINING A MAJOR PROPORTION OF THORIUM OXIDE AND A MINOR PROPORTION OFURANIUM TRIOXIDE WHICH COMPRISES PROVIDING A SMALL AMOUNT OF MOLYBDENUMTRIOXIDE IN SAID SLURRY UNDER SUPERATMOSPHERIC PRESSURE.